Method of insolubilizing gelatin

ABSTRACT

AN AQUEOUS ELECTRICALLY CONDUCTIVE GELATION-CONTAINING SOLUTION IS COATED ONTO A FIRST ELECTRICALLY CONDUCTIVE SUPPORT OR ANODE. THE SOLUTION IS ALLOW TO GEL AND SET. THEN, A SECONDD ELECTRICALLY CONDUCTIVE SUPPORT OR CATHODC IS BROUGHT INTO CONTACT WITH THE GEL TO FORM A SWANDWICH. PASSING A CURRENT THROUGH THE GELATIN-CONTAINING LAYER CAUSES THE GELATIN TO INSOLUBILIZE ON THE FIRST ELECTRICALLY CONDUCTIVE SUPPORT; THE DEGREE OF INSOLUBILIZATION BEING PROPORTIONAL TO THE AMOUNT OF CURRENT PASSED.

United States Patent O 3,575,828 METHOD OF INSOLUBILIZING GELATIN Harvey A. Hodes, James F. Sobieski, and Michael C. Zerner, Eatontown, N.J., assignors to the United States of America as represented by the Secretary of the Army N Drawing. Filed Mar. 1, 1968, Ser. No. 709,827

Int. Cl. B01k 1/00 US. Cl. 204-131 1 Claim ABSTRACT OF THE DISCLOSURE An aqueous electrically conductive gelatin-containing solution is coated onto a first electrically conductive support or anode. The solution is allowed to gel and set. Then, a second electrically conductive support or cathode is brought into contact with the gel to form a sandwich. Passing a current through the gelatin-containing layer causes the gelatin to insolubilize on the first electrically conductive support; the degree of insolubilization being proportional to the amount of current passed.

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION This invention relates, in general, to a method of insolubilizing gelatin and in particular, to a method of selectively insolubilizing gelatin on a flat electrically conductive support.

It is known that various organic and inorganic compounds render gelatin insoluble, or less soluble, in water. Such compounds include formaldehyde, chrome alum and aluminum sulfate. Thus when these compounds are added to an aqueous solution of gelatin that is coated on a substrate and allowed to gel, there results a uniform hardened layer of gel. This uniformly hardened layer is not satisfactory where one desires to selectively control the degree of hardness of the gelatin layer as is the case in certain applications such as electrophotography, photographic engraving and printed circuit manufacturing.

The general object of this invention is to provide a method of insolubilizing an aqueous solution or gel of gelatin. A more specific object of this invention is to provide such a method wherein the degree of hardening of the gelatin can be controlled. A still further object of the invention is to provide a method of selectively insolubilizing the gelatin contained in an aqueous electrically conductive solution or gel thereof on a fiat electrically conductive substrate.

SUMMARY OF THE INVENTION An aqueous, electrically conductive gelatin containing solution is prepared. The solution is then coated onto a first fiat electrically conductive support and the solution allowed to gel and set. A second fiat electrically conductive support is then brought into contact with the gelatin to form a sandwich. When a current is passed through the gelatin containing layer, the gelatin selectively insolubilizes or hardens on the first fiat electrically conductive support in proportion to the amount of current passed.

The method of the invention is based on two general principles.

The first of these two principles is that of the oxidation of metal anodes upon the passage of current in an electrolytic cell.

M- M+ +newhere M is the anode metal, n an integer, generally 1, 2,

3,575,828 Patented Apr. 20, 1971 In general, an ion containing a metallic element, M, produces at the anode metal ion, M, as in the process above but through a different mechanism. The important aspect of the first principle is, of course, the electrolytic formation of metal ions upon electrolysis.

The second principle, is the water insolubilization of gelatin through the use of metal ion complexes of gelatin. Most often the metal ions used are polyvalent, such as Al+ Cr+ Rh+ Pe Th, Zr+ Ti+ and Ce+. The associated negative ion is of some importance; for example, sulfates are particularly effective with Cr+ The insoluble complex form is believed to be of the nature bringing together 2, 3, 4, 5 or 6 gelatin molecules. Another example of an effective anion is chloride ion with 1%, again bringing together 2, 3, 4, 5 or 6 gelatin molecules.

The aqueous, electrically conductive gelatin-containing solution according to the invention will include electrolytes, humectants, and may also include dyes or pigments.

Three groups of electrolytes may be used. The first of these electrolyte groups includes salts containing anions eflicient in dissolving the anode metal if the first principle above is to be used. For example, chlorides are very effective and upon the passage of current aid in dissolving chromium and aluminum anodes. The second group of electrolytes includes salts containing anions that are effective insolubilizing agents in conjunction with the metal ion that is to be dissolved and injected into the gelatin media. Tables of insolubilizing or so-called tanning agents for gelatin are available; the electrolysis supplies the metal ion (cation) the electrolyte supplying the necessary anion. For example, since chromium potassium sulfate (chrom alum) and aluminum potassium sulfate (alum) are both very eflective insolubilizing agents, potassium sulfate is added to the gelatin solution. Electrolysis then supplies either chromium or aluminum, depending on the anode used. A third group of electrolytes can be used and includes salts that are soluble anions containing metal atoms, which upon electrolytic oxidation yield metal ions capable of causing insolubilization. For example, both chromate and dichromate ions yield Cr+ at the anode. The amount of electrolyte in the solution can be adjusted to give the desired resistivity and current sensitivity, and can range from traces to near saturation.

Humectants are added to the gelatin containing solution to maintain the water content of the gelled emulsion. This is necessary to aid in the passage of current through the emulsion upon the application of an electric potential. Humectants maintain ionic mobility and thus conductance. Glycerin is an example of a humectant that is particularly compatible with gelatin. the amount of humectant added generally amounts to 0.1 to 5 percent by weight of the solution.

Dyes or pigments can be included in the gelatin containing solution. If this is done, then after image-wise insolubilization of the gelatin through electrolysis, a simple wash-away process with cold water will leave behind on the substrate a colored or opaque image-Wise resist. If a dye or pigment is included, the amount added will depend on the dye or pigment used and the opacity desired.

Very small amounts of Wetting agents are often desirable in making uniform coatings. Any of the commercially available surface active agents are satisfactory such as Carbide and Carbon Chemical Companys Tergitol series, Dow Chemical Corporations Dowfax and Glyco Products Wetanol.

According to the invention then, the aqueous electrically conductive gelatin containing solution is coated onto a flat electrically conductive support or anode and allowed to gel and set. This substrate can be a metal plate or foil. It must, of course, contribute the necessary metal ions upon electrolysis. The substrate can also be a sheet of plastic, as for example Mylar, with a thin metallic coating. After a second flat electrically conductive support or cathode is brought into contact with the gel to form a sandwich, a current is passed through the gelatin containing layer causing the gelatin to insolubilize on the anode, the degree of insolubilization being proportional to the amount of current passed.

DESCRIPTION OF THE PREFERRED EMBODIMENT An aqueous solution of percent by weight gelatin containing 0.5 percent glycerin, 0.2 N KCl, 0.25 percent wetting agent, 0.02 N K 50 and nigrosene as a black dye, was coated onto steel and aluminum plates which served as anodes. Plates were then brought into contact with the gel to form a sandwich. These plates served as cathodes. An application of 1 /2 volts or greater was sufiicient to cause a gelatin resist on the anode plates. The passage of 05x10- coulombs over an area of 1 square centimeter leaves a faint 1 cm. resist.

The method of the invention as described herein to cause the insolubilization or the so-called tanning of gelatin can be used in various electrophotographic applications. In one such application, the gelatin emulsion layer on its electrically conductive support or anode constitutes the image recording layer or the film. The film is placed in contact with a photoconductive layer or cathode. Upon the applictaion of an electric potential, current will flow through the film, the amount of which will be directly proportional to the amount of illumination that falls on the photoconductor. Such image forming devices are disclosed and claimed in US. patent application of Hodes, Sobieski and Zerner for Photoelectric Imaging Device, Ser. Nos. 670,815 and 670,816 filed Sept. 26, 1967 and assigned to the same assignee. In such devices, if an image is focused on the photoconductor, then current will flow in an image-wise manner. The photoconductor is then removed from the film, and the film washed in cold or lukewarm water. An image-wise resist remains. If there is a dye or pigment incorporated in the film, this image will be immediately apparent.

Anode oxidation and solution, described here to cause insolubilization of gelatin has also been used as a method for causing electrolytically initiated polymerization, as disclosed and claimed in US. patent application of Hodes, Sobieski and Zerner for Method of Initiating Polymerization Electrolytically, Ser. No. 681,062 filed Nov. 3, 1967 and assigned to the same assignee. If the method of the instant invention is combined with the method of the Ser. No. 681,062 application into a single process, the resulting film will be more current sensitive than either method individually. In such a case, gelatin is used as a binder for the emulsion of organic monomers as described in Ser. No. 681,062. After polymerization, the gelatin binder is washed in warm water. Where current passed, there will be both polymer and insolubilized gelatin. The metal substrate or anode is an alloy containing a tanning metal and a metal capable of causing polymerization. Steel is an excellent example. In this case, dissolved iron causes polymerization upon processing, and dissolved chromium is extremely effective in causing gelatin insolubilization or tanning.

The foregoing is to be considered merely as illustrative and not in limitation of the invention as hereinafter claimed.

What is claimed is:

1. A method of insolubilizing the gelatin contained in an aqueous solution of gelatin to cause a gelatin resist to form on a plate including the steps of;

(1) preparing an aqueous electrically conductive solution of 10 percent by weight of gelatin containing 0.5 percent glycerin, 0.2 normal potassium chloride, 0.25 percent wetting agent, 0.02 normal potassium sulfate, and nigrosine as a black dye,

(2) coating the gelatin solution onto a steel anode plate,

(3) bringing a cathode plate into contact with the gel to form a sandwich, and

(4) passing a current through the gelatin containing layer to cause a gelatin resist on the anode plate.

References Cited UNITED STATES PATENTS 1,915,568 6/1933 Gortner et al. 204l3l 1,199,457 9/1916 EllithOrp 204137 3,468,303 9/1969 Mosier 20456 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204131 

